As is known, the elements which are most actively involved in the operation of a sewing machine of the industrial type consist, in addition to the needle, of a lower looper which oscillates below the fabric being sewn and an upper looper which oscillates in opposite phase relative to the lower looper, on either side of the edge of the fabric itself. These basic elements can have different shapes and structures, depending on the stitches which must be made. In particular, it is possible to make stitches using two or three threads which are attached to the needle, lower looper and, if necessary, upper looper.
The present invention relates in particular to the formation of an overedge stitch with three threads, classified under No. 504 in the Federal Standard Catalog. Moreover the present invention relates in particular to the structure of the abovementioned upper looper which, in the case of this specific application, has attached to it a thread and is therefore provided with a hole at its active end, through which the thread itself passes. This thread cooperates with the thread of the needle and the thread of the lower looper so as to form the abovementioned stitch which, as can also be seen from the attached drawings, is a stitch arranged on either side of the edge of a fabric.
Moreover, it is worth noting that, for the purposes which come within the scope of the present invention, provision is made, obviously by way of example, for the use of fabrics with a relatively high thickness, where required. In the prior art, the abovementioned upper looper was substantially capable of performing the function for which it was assigned during formation of the stitch in question, but had numerous drawbacks which gave rise to the occasional loss of stitches and/or to an operating speed which was not particularly high. So as to overcome these drawbacks, the Applicants have devised new technical solutions which provide the upper looper with maximum reliability during operation. In particular, the present invention deals with the technical problem of how to transfer the thread of the lower looper onto the upper looper in question, in a constant and practical manner, when the latter is moving towards the fabric.
It is known that the trajectory of the upper looper is initially almost vertical and that, during this section of the trajectory, the upper looper moves towards the lower looper and inserts itself between the latter and the associated thread. Subsequently, so that the stitch can be formed correctly, the upper looper must allow the thread of the lower hook to run along its own blade. Sometimes, however, it may happen that the thread of the lower hook gets caught on the front part of the upper hook, instead of running along the blade of the latter. Furthermore the rectilinear profile of the upper looper allows, under some unfavorable circumstances, an excessive running of said thread of the lower looper, after the first impact on said tip, far beyond the blade and up to the shank. This happens, in general, on account of the abovementioned trajectories of the said upper looper and owing to the specific technical and dimensional properties of the blade of the upper looper itself. The general aim of the present invention is to design an upper looper provided with a hole, which is able to overcome the abovementioned drawback, whilst fulfilling the said technical function, and also improve the reliability and operating precision of the upper looper. Within the context of this general aim, an important object of the present invention is to design an upper looper provided with a hole and with a structure which is particularly simple and easily produced by the industries in the sector.
These and other objects which will become clearer below, are substantially achieved by means of an upper looper of substantially rectilinear form and provided with a hole and used for making an overedge stitch with three threads in overlock sewing machines of the industrial type, wherein said looper comprises, at its end provided with a hole, a tip which is curved in the direction of the lower looper, a rounded and bulged projection adjoining the said curved tip, which rounded and bulged projection faces the stitches already formed and is provided, on its upper edge, which an inclined chamfer adapted to facilitate the transferring of the thread from the lower looper onto said upper looper, and wherein the substantially rectilinear profile of said upper looper is interrupted by a substantially vertical and upwardly projecting lug designed to define a guide and stop member for said thread of the lower looper; said lug being disposed substantially midway between the blade and the shank of said upper looper.